tudies carried out under this project focus on the structure nd function of mammalian retroviruses. A perplexing feature f the envelope gene associated with human endogenous etroviral DNA has been its unrelatedness to known infectious ammalian retroviruses. These cloned env segments fail to ybridize to clones of baboon endogenous virus or Moloney eukemia virus under any hybridization condition. During the ast year, however, we have demonstrated that the human env DNA egment reacts strongly with orangutan, baboon, chimpanzee, frican green monkey, and, of course, human chromosomal DNAs; o hybridization could be detected with mouse DNA. This result ndicates that baboon genomic DNA, for instance, contains at east two major classes of endogenous sequences: one that ives rise to the infectious BaEV and a second that is very losely related to human endogenous retroviral segments. Based n this result, we have rescreened a human gene library under elective hybridization conditions and have identified several lones that belong to a second major class of endogenous etroviruses. Work is in progress to establish the structural nd functional relationships of the two classes of endogenous uman retroviral sequences. series of oligopeptides whose amino acid sequence was deduced rom the primary nucleotide sequence of both murine and human etroviral DNAs was synthesized. Based on our ability to etect retroviral-specific RNA in human placentas, colon arcinoma, and breast carcinoma cells, we have raised antibody gainst synthetic oligopeptides whose primary sequence was educed from different portions of the human retroviral env egion. A 83K polypeptide has been identified in several ifferent human cell lines; this putative retroviral gene roduct is particularly prominent in colon carcinoma cells. he reactivity of the rabbit antibody with the human env olypeptide can be blocked by prior incubation with the human etroviral envelope oligopeptide.